The overall objectives of this project are divided into three areas: 1) Studies relating to the localization, characterization and purification of the mineralocorticoid receptor (aldosterone-binding protein). 2) Studies relating to the mechanism and regulation of sodium transport in the urinary bladder of the toad. 3) Studies relating to the mechanism and regulation of urinary acidification by the urinary bladder of the toad. In the current year the studies were directed at the effect of iodination of the mucosal surface of the toad bladder: peroxidase catalyzed iodination inhibited urinary acidification but produced no effect on Na transport nor upon the stimulation of Na ion transport or osmotic water permeability by vasopressin. Studies on the mineralocorticoid receptor in a dog kidney cell line were continued. BIBLIOGRAPHIC REFERENCES: Anderson, N.S., III and D.D. Fanestil. Corticoid Receptors in Rat Brain: Evidence for an Aldosterone Receptor. Endocrinol. 98:676-684, 1976. Ludens, J.H., D.A. Vaughn and D.D. Fanestil. Bicarbonate-Stimulated ATP-ase and Acidification in Toad Urinary Bladder. Clin. Res. 24:162A, 1976.